1. Field of the Invention
This invention generally relates to superconducting structures and their fabrication. In particular, this invention relates to low loss superconducting films on alkaline earth fluoride substrates.
The invention also generally relates to metal oxide structures. More particularly, this invention relates to a metal oxide film grown on a single crystalline silicon substrate with a buried oxide layer, and to free standing films formed therefrom.
This invention also generally relates to a substrate fixture assembly which is particularly useful for pulsed laser deposition. More particularly this invention relates to a substrate fixture providing simultaneous double sided coating and simultaneous multiple substrate coating.
2. Discussion of the Background
Low loss superconductive components can make vital contributions to current and future microwave and millimeter wave systems. There have been significant efforts to develop high temperature superconducting thin films (HTSC) for applications in the micro-wave components. The HTSC thin films used in typical micro-wave components must be deposited on a micro-wave compatible substrate having a low dielectric constant (.epsilon.) and a low loss tangent in order to avoid unacceptable power dissipation in the substrate.
Of all the metal-oxide superconductors, thin films of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x for x=O.5 to 1.0 (YBCO) are the most technologically advanced. State of the art YBCO films on LaAlO.sub.3 substrates have achieved the best micro-wave performance. For example, pulsed laser deposition (PLD) deposited and patterned YBCO films on LaAlO.sub.3 have shown surface resistance values lower than that of Cu by a factor of 20 at X-band (X-band spans 8-12 Ghz) and lower than that of Cu by more than two orders of magnitude at L-band (below 5 Ghz). However, LaAlO.sub.3 has a dielectric constant (.epsilon.) of 25.
While a variety of applications at the lower end of the micro-wave spectrum could be realized with current HTSC technology, satellite communications, radars and missile seekers all require higher frequencies in the millimeter wavelength region. In order for superconductor films to be useful at those higher frequencies substrate dielectric losses must be reduced, which requires a substrate with a low dielectric constant.
At high frequencies superconductors are not completely lossless. When a superconductor is used as a high frequency line to transport a signal, the resistive loss associated with that line decreases as the line width increases. Therefore, it is desirable to have relatively wide lines in order to reduce resistive loss. However, dielectric loss is proportional to the dielectric constant of a material and to the volume of dielectric material through which a high frequency electromagnetic wave is transported. Therefore, when the width of a superconducting line on a dielectric substrate is increased, the dielectric loss is also increased. Therefore it is useful to lower the dielectric constant of the dielectric material in which a superconducting line is formed in order to achieve to a superconducting waveguide which has the lowest possible high frequency loss.
A report prepared for the GE Astro Space Division by Belohoubek et al (Dec. 89) on "Applications of high T.sub.c superconductors for satellite communication systems" points out that even at the C-band frequencies (3.6-6.2 gigahertz) a low dielectric constant is important for a substrate because substrate dielectric loss becomes important for light weight, high performance filters. The relatively large dielectric constants of LaAlO.sub.3 (.epsilon.=25), Sapphire (.epsilon.=11) and MgO (.epsilon.=9.5) impose restrictions on the possible use of those substrates in conjunction with the high quality YBCO films, at least at millimeter wave frequencies. By a high quality HTSC film is meant an HTSC film which has a surface resistance at 10 gigahertz of less than 10 milli-ohms and preferably less than 1 milli-ohm.
Alkaline earth fluorides with the general formula MF.sub.2 (where M=Mg, Ba, Sr, Ca) have static dielectric constants between 5 and 7 at liquid nitrogen temperatures (77K) and have been used as substrate material for YBCO films. However, YBCO films grown on MF.sub.2 substrates have poor superconducting properties, such as the sharpness of the normal to superconducting transition and resistance versus temperature, as reported by P. Madakson et al. in Journal of Applied Physics, volume 63, No. 6, pp. 2046 (1988), and Siu-Wai Chan et al. in Applied Physics Letters, Volume 54, No. 20, pp 2032 (1989). Those YBCO films are also polycrystalline thereby introducing additional energy loss mechanisms at high frequency. Therefore those films are not suitable for micro-wave and millimeter-wave applications.
There have also been attempts to deposit YBCO films on top of a CaF.sub.2 buffer layer buffering a GaAs substrate, as reported by K. Mizuno et al in Applied Physics Letters, Vol. 54, No. 4, pp. 383, (1989). The quality of those YBCO films is rather poor relative to the film quality when YBCO is deposited on high quality substrates such as LaAlO.sub.3. There is a continuing need for superconducting thin films on substrates which provide lower energy loss at high frequencies than is now possible.
Another need is for improved infra-red (IR) detectors. It is known that superconducting films may be used for IR detectors. Low heat capacity substrates which have high thermal conductivity are desired for superconducting IR detectors in order to increase detector signal and speed. One substrate which has very high thermal conductivity is diamond. However high quality high T.sub.c films can not be deposited on diamond substrates due to lattice and thermal expansion mismatch and due to reaction of diamond with oxygen.
HTSC films have been deposited upon buffered reactive substrates by depositing a buffer layer on the reactive substrate prior to depositing an HTSC film. By a reactive substrate we mean any substrate upon which a thin film of a HTSC substantially interacts with the substrate so that the HTSC film is not superconducting or does not have a superconducting transition temperature or as high a critical current density within about a hundred angstroms of the interface between the HTSC film and the substrate.
In all the deposition schemes, there remains the difficulty of preparing a superconducting film which does not degrade due to its substrate and a substrate whose properties do not detract from the desired function of a superconducting device into which it is incorporated.
Many thin films have been deposited by PLD. During PLD, pulses of an ultraviolet laser are directed onto a target material. Each pulse vaporizes a small amount of the target surface. The vaporized material interacts with both the laser beam and target surface to create a dense plume of material rapidly travelling directly away from the target. A center of momentum of the plume is directed along the target surface normal so that a substrate surface is usually positioned opposing the target surface in order to block the plume. The plume impinges upon the substrate and deposits thereon to form a film. The material in the plume travels mainly directly away from the surface along the target surface normal. The plume also usually contains particles on the order of 1000 angstroms to 10 micron.
HTSCs such as YBa.sub.2 Cu.sub.3 O.sub.7 are typically deposited onto single crystalline substrates held at a temperature between 600.degree. and 850.degree. C. Many applications require HTSC films deposited with a substrate surface temperature uniformity of .+-.5.degree. C. for optimal growth of the desired phase. For single-sided coatings of substrates of diameter of less than 1 inch, substrates have been adhered to a hot plate with a high thermal conductivity paste in order to provide the desired temperature control. Radiative heaters have been used to heat the back sides of larger substrates, i.e., the side of the substrates that faces away from the target.
The large particles ejected from a target during PLD impinge upon and remain on the substrate. For many applications, those particulates are catastrophic. Also, for many technologically important applications, it is necessary to deposit buffer layers or create multilayer structures, which are degraded by the large particles in PLD films. Present substrate fixtures for PLD allow only single sided coating of substrates and are only suitable for coating a single substrate at a time. While such substrates are acceptable for research, they severely limit the commercial applications because of low throughput and the restriction of single side coating. Clearly PLD systems have several disadvantages limiting their usefulness for manufacturing thin film devices. A need exists to overcome all those disadvantages if PLD is to ever become commercially important.
Herein, perovskite crystal structure type material means any material which has the CaTiO.sub.3 cubic crystal structure type or any of the tetragonal and rhombohedral or orthorhombic distortions of that cubic crystal structure type.
Herein, by epitaxial is generally meant that a layer grown on a single crystal substrate surface is also single crystal and has a particular crystallographic relationship to the substrate surface and that that crystallographic relationship is determined by the crystallography of the substrate surface and the material and deposition conditions of the grown layer. Particular crystallographic relationships either indicating alignment of crystallographic axes of a substrate with crystallographic axes of a film or alignment of crystallographic axes of substrate and film and also constraint of film lattice constants to be the same as lattice constants of the substrate at the film/substrate interface are disclosed for various interfaces throughout this application.
As indicated by the foregoing discussions, high temperature superconducting film technology has not progressed to a point enabling large scale integration of multiple functions on a single substrate.